In the thermal power generation system, there has been a tendency to drastically increase the steam temperature of the steam turbine in order to enhance the generating efficiency. As a result, the required high temperature characteristics become more strict. Many materials for use in such application have hitherto been suggested. Amongst them, it has been known that the development heat resisting steels suggested in JP-A-2-290950 (the term "JP-A" used herein means an unexamined Japanese patent application) and JP-A-4-147948 (the components used are the same but the intended uses are different from each other) are excellent in high temperature strength.
However, in order to further enhance the power generation efficiency for use in raw materials for turbines, the above-mentioned development heat resisting steels do not yet have sufficient high temperature characteristics, and heat temperature characteristics including high temperature creep strength need to be further enhanced. Moreover, the conventional materials are also problematic in that their toughness is reduced by long-time aging at high temperature and, thus their durability is poor. It has been desired to improve the characteristics of the heat resisting steels including the characteristics described above.
We have carried out the improvement in the abovementioned heat resisting steels in light of the following viewpoints in order to make it possible to highly enhance the generating efficiency and enhance durability:
(1) Enhancement of high temperature creep strength
(2) Prevention of deterioration of toughness by long-time aging at high temperature
(3) Enhancement of toughness
As a result of our studies, the following means are available for attaining the above objects:
(1) The enhancement of high temperature creep strength can be realized by containing Nb, Ta and B and decreasing the Mn content.
(2) The prevention of deterioration of toughness by long-time aging at high temperature can be realized by decreasing the contents of Si, Mn, P, As, Sn and Sb
(3) The enhancement of toughness can be realized by containing a rare earth element and Ca and decreasing the S content.
The present invention has been done based on the above circumstances, and an object of the present invention is to provide a heat resisting steel having excellent high temperature characteristics and durability by enhancing the high temperature creep strength, preventing the deterioration of the toughness by long-time aging at high temperature and enhancing toughness.